The present invention relates to compositions and methods for cleaning and passivating stainless steel surfaces, such as gas flow equipment, pharmaceutical manufacturing equipment, and semiconductor processing equipment.
During the past fifteen years the requirements for cleanliness in semiconductor processing equipment have increased at least a hundred times. Semiconductor feature sizes have been cut in half in the past few years and packing densities have doubled or tripled in the same time period. It also appears that the rate of change is accelerating rather than holding at past rates. With these changes, the problems caused by contamination in semiconductor processing become even more serious. Cleanliness is also important in the health and pharmaceutical industries, driven by the need to reduce the contamination of treatment processes.
In the past, stainless steel equipment used in these processes have been cleaned almost universally by use of solvents. In addition to the problems of atmospheric pollution and operator health hazards, solvents do not clean absolutely. They leave films and particle residuals. Ultrasonic cleaning may also drive particles into crevices in instrument parts, for a later release. Chlorofluorocarbon cleaning solvents sold under the trademark Freon are examples of known cleaning solvents as well as 1,1,1-trichloroethane and methylene chloride.
The lack of cleanliness of the components cleaned by conventional solvents, methods and apparatus is problematical where active ions and organic contamination such as organic films remain on the components. Active ions, e.g. metallic ions, can adversely affect the process in which the equipment is to be used.
Passivation of cleaned steel surfaces is important for preventing conditions such as flash rusting of cleaned wet steel.
In the prior art, cleaned steel is often passivated by treating with a nitric acid solution to provide altered surface characteristics that resist rusting. Dilute solutions of citric acid made alkaline with ammonia or with an amine have been used for passivation of cleaned steel surfaces. These same solutions also have been used in combination with sodium nitrite.
Water-soluble amines are sometimes added to latex or water-dispersed coatings for steel to reduce corrosion. Water-soluble amines also have been added to final rinses for cleaned steel, but always in combination with other materials (such as other alkaline chemicals, citric acid, sodium nitrite, etc., and as exemplified in U.S. Pat. Nos. 3,072,502; 3,154,438; 3,368,913; 3,519,458; and 4,045,253) and therefore these rinses have left insoluble residues on the steel surfaces that are detrimental to optimum performance of subsequently applied protective coatings.
In the prior art, cleaned steel is often passivated by treating with an alkaline sodium nitrite solution to provide altered surface characteristics that resist rusting. For unknown reasons, this method is sometimes ineffective for passivating cleaned steel.
Dilute solutions of citric acid made alkaline with ammonia or with an amine have been used for passivation of cleaned steel surfaces. These same solutions also have been used in combination with sodium nitrite.
U.S. Pat. No. 4,590,100 describes a process that allows previously cleaned steel to be passivated with a rinse of almost pure water, that is made slightly alkaline with an amine to inhibit corrosion preparatory to application of non-aqueous protective coatings, such that any small amine residue remaining on the steel surface after drying of the water will itself evaporate and in such a manner that any remaining amine residue will be incorporated into the non-aqueous protective coating without leaving any water-soluble or ionic residue on the surface of the steel.
U.S. Pat. Nos. 5,252,363 and 5,321,061 describe aqueous organic resin-containing compositions which are useful for depositing coatings on freshly galvanized metals to protect the metals against white rust and provide a surface which is universally paintable. The organic resin consists essentially of at least one water-dispersible or emusifiable epoxy resin or a mixture of resins containing at least one water-dispersible or emulsifiable epoxy resin.
U.S. Pat. No. 5,039,349 describes a method and apparatus for cleaning surfaces, such as semiconductor processing equipment and pharmaceutical processing equipment, to absolute or near-absolute cleanliness involving spraying jets of heated cleaning solution so that it flows over and scrubs the surfaces to be cleaned, producing a rinse liquid. The rinse liquid is filtered and recirculated over the surface to be cleaned.
It is a purpose of the present invention to provide alkali-based formulations which both clean and passivate stainless steel surfaces.